1. Field of the Invention
This invention relates to a length measuring apparatus for measuring the relative position of two objects movable relative to each other. The apparatus includes a scanning unit attached to the first object capable of scanning the graduation of a scale attached to the second object. This apparatus is especially useful for measuring the position of a tool relative to a workpiece. The apparatus is also useful in coordinate measuring machines for determining position and/or dimensions of test objects.
2. Prior Art
German unexamined patent specification OS No. 2,853,771 discloses a length measuring arrangement for measuring the relative position of a first and second object movable relative to one another, in which a scanning unit fastened to the first object scans the graduation of a scale on a carrier. The carrier is joined at both ends to the second object by means of fastening elements. One end of the carrier is connected directly to the second object with a fastening element. At the other end, a length compensation element deflectable only in the direction of measurement is positioned between the carrier and the fastening element. This length compensation element is formed as a plate which is subdivided by a U-shaped slit into an inner part and an outer part. The inner part projects in the measuring direction while the outer part is rigidly positioned, being joined via the fastening element to the carrier. The length compensation element allows translatory movement of the other end of the carrier in the measuring directoon, eliminating longitudinal forces resulting from thermally-induced length changes of the carrier. As a result, different thermal expansion coefficients of the carrier and the second object will not affect measuring accuracy. A fixed measuring reference point on one end of the scale is effected with respect to the second object by the direct fastening of the one end of the carrier of the scale to the second object using a fastening element.
German patent No. 3,2l9,O83 discloses a similarly constructed length measuring system in which the length compensation element at the second end of the carrier is integrated into the fastening element. A spring element associated with the fastening element comprises meander-form slits running in planes substantially perpendicular to the measuring direction.
German unexamined patent specification OS No. 3,242,966 discloses a length measuring system for measuring the relative position of a first and second object movable relative to one another in which a scanning unit fastened to the first object scans the graduation of a scale on a carrier. The carrier of the scale is connected to the second object via fastening elements located at both ends and about the middle of the carrier. The fastening elements at both ends are associated with length compensation elements to allow translatory movement of both ends of the carrier in the direction of measurement. The fixed measuring reference point with respect to the second object is established by fastening the center of the carrier of the scale directly to the second object using a fastening element. By allowing translatory movement of the ends of the carrier in the measuring direction, the length compensation elements eliminate longitudinal forces evoked by thermally-induced length changes of the carrier. As a result, different thermal expansion coefficients of the carrier and the second object will not affect measuring accuracy. The two length compensation elements each consist of a spring element comprising meander-form slits running in planes substantially perpendicular to the measuring direction. The length compensation elements are integrated into the fastening elements.
In the above-mentioned length measuring arrangements the fixed measuring reference point of the scale with respect to the object to be measured is established during manufacturing and cannot be varied by the user. The preferred position of the measuring reference point on the scale, however, varies both with the type of processing machine and with the place of attachment of the measuring apparatus on the machine parts of each processing machine. Different machine parts whose relative position is to be measured generally exhibit different temperature behavior whether the different parts are located in the same or different processing machines. The user, in ordering such a length measuring arrangement, must select a fixed measuring reference point on the scale suitable for the specific place of attachment to the specific processing machine. Use of the same length measuring arrangement in different places of attachment is not always possible because of the generally different temperature behavior.